The present invention relates to a pneumatic hammer mechanism, in particular an electrically driven, pneumatic hammer mechanism, for a power tool, in particular a hand power tool, e.g., a chipping hammer.
An electrically operated chipping hammer having a pneumatic hammer mechanism is known from European Patent Document No. EP 1 779 980 A2 among others. A schematic representation of its hammer mechanism 501 from FIG. 6 is incorporated as FIG. 1.
A flying mass 569 is arranged in a piston cylinder 530 between a hammer piston 520 and an end piece of a tool 599. The flying mass 569 and the hammer piston 520 make an airtight seal with a wall of the piston cylinder so that a sealed airtight chamber 580 is formed between the flying mass 569 and the hammer piston 520. The chamber 580 will be called pneumatic chamber 580 in the following.
The hammer piston 520 moves periodically in a reciprocating manner in the piston cylinder 530, driven by a gear wheel 522, 523, 531. The flying mass 569 is also excited to move periodically between the hammer piston 520 and the end piece of the tool 599 based on its coupling to the hammer piston 520 by means of the pneumatic chamber 580.
FIG. 2 schematically shows the progression of movement of the hammer piston 520 and flying mass 569 over time t; the progression among other things is also depicted in FIG. 13A of EP 1 779 980 A2. The local axis x indicates the distance from the end piece of the tool 599. When the hammer piston 520 moves at its greatest velocity in the direction of the tool 599 (at small x values), the hammer piston 520 and the flying mass 569 come as close as possible. The pneumatic chamber 580 is heavily compressed in the process and as a result accelerates the flying mass 569 in the direction of the tool 599. After this, the flying mass 569 strikes undamped the end piece of the tool 599. A portion of the kinetic energy of the flying mass 569 is transferred in the process to the tool. As with a partial elastic impact with a heavy impact mate, the flying mass 569 reverses its direction of movement and moves with reduced velocity in the direction of the hammer piston 520. The stroke H of the hammer piston 520, the angular velocity of the hammer piston 520 and the maximum length of the pneumatic chamber 580 are coordinated with each other such that the movement of the flying mass 569, as depicted, is excited resonantly by the hammer piston 520.
There is the need to further increase the impact effect of the chipping hammer without increasing the power consumption of the chipping hammer in the process. The impact effect of the chipping hammer is produced essentially from the energy released by an impact in a work piece. The power consumption is yielded from the product of the energy released per impact and the impact frequency of the impacts. Consequently, the impact frequency of the impacts must be reduced.
The energy released by each impact depends upon the kinetic energy that the flying mass 569 collects up until impact. The acceleration work is performed by the hammer piston 520, which increases with increasing velocity of the hammer piston 520 in the piston cylinder 530. The velocity of the hammer piston 520 is predetermined by the angular velocity and the stroke H of the hammer piston 520. Even though increasing the angular velocity based on the impact frequency of the impacts that increases with it is not suitable, the stroke H of the hammer piston 520 can be increased. However, this requires a greater maximum length of the pneumatic chamber 580 and thus a longer hammer mechanism in order to guarantee a resonant excitation of the flying mass 569.
So that a user may hold the chipping hammer ergonomically during operation, the dimensions of the chipping hammer and thus also of the hammer mechanism are restricted, however.
The kinetic energy of the flying mass 569 can also be achieved by increasing its mass, however, an operator then experiences a greater recoil during acceleration of the flying mass 569 from the hammer piston 520.
One objective is making a percussive power tool available that facilitates an improved impact effect taking ergonomic aspects of into consideration.
The hammer mechanism features: a flying mass, which is movable along an impact axis; an impact surface, which limits a movement of the flying mass along the impact axis in the impact direction; an exciting piston, which limits a movement of the flying mass along the impact axis opposite from the impact direction; a pneumatic chamber between the flying mass and exciting piston; a drive for periodically moving the exciting piston with a stroke H along the impact axis, wherein the flying mass is excited to a periodic movement between the impact surface and a minimum approach of the exciting piston. In this case, the following inequality applies for the mass m2 of the flying mass, a cross-sectional area A of the pneumatic chamber, the maximum length L of the pneumatic chamber, the stroke H of the exciting piston and an impact coefficient q, if the hammer mechanism has an impact frequency f during percussive operation:
                              L          κ                          2          ⁢                                    (                              L                -                H                            )                        κ                              ·              κ                  L          -          H                      +                            (                                                    L                κ                                            2                ⁢                                                      (                                          L                      -                      H                                        )                                    κ                                                      -            1                    )                ·                              1            -            q                    q                    ⁢              N                  2          ⁢          π          ⁢                                          ⁢          H                      ⁢      ≥    !    ⁢                              m          2                          A          ·                      p            0                              ·              N        2              ⁢          f      2      wherein the parameter N is at least 4, po designates the ambient pressure and κ the isentropic coefficient of gas in the pneumatic chamber.
The maximum length of the pneumatic chamber is the distance of the exciting piston from the flying mass, when the exciting piston is arranged in its position away from the tool receptacle and the flying mass is arranged adjacent to the impact surface. The maximum length is used as the value to design and characterize the hammer mechanism. During operation, the pneumatic chamber as a rule does not occupy the maximum length at any point in time.
The impact coefficient q designates the ratio of the velocities of the flying mass after the impact to before the impact. The impact coefficient is determined essentially only by the masses and shapes of the flying mass and the impact body.
One cycle of the flying mass in the hammer mechanism is made up of a first phase with a movement from the minimum approach of the exciting piston to the impact and a second phase with a movement from the impact position to the next minimum approach of the exciting piston. The first phase and the second phase are completed together within a period of time, which is predetermined by the cycle duration of the movement of the exciting piston. Due to the deceleration of the flying mass until the momentary standstill, the duration of the second phase increases to the detriment of the duration of the first phase. The flying mass overcomes the distance between the minimum approach and the impact in a shorter time, ergo, as desired, with a higher velocity.
The deceleration of the flying mass during the second phase takes place if the dimensions of stroke and maximum length of the pneumatic chamber are suitably selected. The pneumatic chamber is compressed at the beginning of the second phase, because after the impact the exciting piston is still moving in the impact direction or the flying mass is initially moving with a greater velocity against the impact direction than the exciting piston. In this connection, an increase in pressure is produced in the pneumatic chamber, which decelerates the flying mass. The increase in pressure is all the greater, the smaller the volume of the pneumatic chamber or the greater the still remaining stroke movement of the exciting piston is in the direction of the impact surface.
Based on hammer mechanisms that have been realized and numeric simulations, it was recognized that with typical parameters with respect to the mass of the flying mass, a diameter of the pneumatic chamber and an impact frequency, in operation the cited ratio of 1.55 achieves an increase in the impact energy based on a slow movement of the flying mass in the second phase.
One embodiment of the invention provides that the stroke is selected as a function of the maximum length of the pneumatic chamber such that the flying mass changes the direction of movement at least once during the movement between the impact surface and a following minimum approach of the exciting piston. A ratio of less than 1.50 can be advantageous for this. A change in the direction of movement during the second phase produces a longer path, which the flying mass covers during a cycle. The velocity of the flying mass is higher during the first phase, even taking the basic condition of the predetermined period of time for a cycle into consideration.
One embodiment provides that the stroke is selected as a function of the maximum length of the pneumatic chamber such that the flying mass touches the impact surface at least twice between two successive minimum approaches of the exciting piston. A ratio of less than 1.40 can be advantageous for this. The reversal of the direction of movement through the second impact produces a high velocity of the flying mass at the end of the second phase. The flying mass is thus able to closely approach the exciting piston and afterward experiences a greater acceleration in the direction of the impact surface due to the pneumatic chamber.
One embodiment provides that if the mass of the flying mass is greater than 400 g, the length ratio is selected as less than 1.55 and if the mass of the flying mass is less than 400 g, the length ratio is selected as less than 1.40.
One embodiment provides that if a ratio of the mass of the snap die to the mass of the flying mass is less than 1.2, the length ratio is selected as less than 1.40.
The following description explains the invention on the basis of exemplary embodiments and figures.